Method of recovering oxalates



Oct. 9, 1928. 1,687,480 G. H. BUCHANAN ET AL METHOD OF RECOVEBINGOXALATES Filed Sept 4, 1924 (5014 (0mm? J arm/rm mm IN VEN TORS.

BY mm,

.4 TTORNEK Patented Oct. 9, 1928.

UNITED STATES PATEN'T- omcs;

GUY H. BUCHANAN, b1 "ESTFIELD, NEW JERSEY, AND GEORGE BARSK Y, OF NEW.YORK, Y.. ASSIGN 01(5) TO AMERICAN CYANAMID COMPANY, OF NEW YORK, N.

A CORPORATION OF MAINE.

-mmnon or nncovmms OXALATES;

This invention relates to 'the manufacture of oxalates or oxalic acidand more particula'rlr itis directed toimproring the grade of crudeox'alatesi In the copending application 035 Buchanan) Barsky ant Ashley,tiled Deni 18th, 1923', Se rial $0. 6815446, for a process (if-producingsalts of carboxylic acids and assigned tolthe American CyanamidCompany,there is described 01 new'method-of producing oxalates and oxalic acid.This, method consists essentiallyin providing a mixture of a cyanainidand a cyanid, specifically the Caleimnsinlts thereof; and heating'the'same for "a conside'rable length" of time at a temperature corresponding to a steam pressureof about 1C0 pounds per square inch wherein.a reaction takes place in \rhich practically all of the nitrogen isrecovered as ammonia while producinglarge .amounts of .calcium Vforrnateand calcium oxalate. v r The formate' being-soluble, and the oxala'tebeing inso1uble,,t he maybe-readily separated by filtration." Theoxalate present n the 7 resulting filtercake contains large amounts-ofinsoluble impurities snrh as calcium' carbonate and calciunr hydroxide.In many cases the amounts of such impurities may constitute 80% of thetotal mass;-. In ordcr'to recorer oxalic acid therefrom, the mixture maybe tr ated with amineral acid. such as sul-c phuric ac id, butttlieamount th eof necessary for the 'deconmosihon of the oxalate -1srelatively large becausesullicient acid must be used 'to tlecon'lposethe carbonate and the hydroxide as well.

[t is. therefore, among the objects of thisinvention to provide animproved process of' ma'kin; oxalates and oxalic acid, which shalltender umieeefssary larger amounts of acid and thereby decrease the costof producing (lime substances.

i is a further object of our invention to' produce a. high grade oxalateproduct from a material containing small percentages of oxalate.

'1: have discovered that calcium oxalate may l derumposed by sodiumcarbonate under coudit l(lll. such that a complete separa- !ion of theoxalate l'r rr-z the'carbonate and hydroxide may be eIfectedQ Thisdeco:iiposi- The sodium oxalate \\'l llcll isltll'ineil-is. olution'maybe represented by the following bleandinzn be separated fromith eicalcium carbonate by filtration. The solutioxr thns obtainedcontainsallhtheoxalate 4 sodium oxalate; togethclfu'itli \lfbtltOYt-[lesssod-iu'm carbonate \frals used. in} he. ext-Fa ct ion a n l other,soluble salts. either or result ing from the i-eatti'oma. ,1; i i The'proport" equation is necessary. :At 100 .CJ an excess of If); to issutfici'ent, and at higher temperatures a: still. smallerexcessis'rerequiregl r The practical -application-ofour invention may heillustrated in the. treatment of a 'mixture containing calcium oxalate'andca'leimn carbonate. with a large proportion of the' latter compound,to produce a mixture containing a large proportion of calcium oxalate,and a small proportion 'ofcalcinm carbonate. Such mixture beforetreatmcnbmay contain 20); calcium oxalate as the hydrated salt, CaC;,();ll -;an l 80% calcium carbonate, CaCO \Ve .may make 'a slurry 'of suchmixture with water, using suflicient water-to produce a 3% solution ofsodium oxalate, assuming that we secure cornplete-decomposi tion of thecalcium oxalate. Toth'is slurry we add an amount of sodium carbonate,lx'mCU ,equivalentto anexcess of from 15% to over thetheoretical amountrequired to react jx'vith the calcium oxalate according to the followingequation: CaC O .1I O+Na CO V NZl C O,+CaCO,-+H O;

\Ve um v agitate. and heatthis mixture at preseugoriginally f 'the.boiling temperature until all the calcium i oxalate ha been decomposed,as may boas-- certaincd by testing a portion of the mixture atintervals. \Yhen all the calcium oxalate has been decomposed we filterthe mixture hot and obtain a solution of sodium oxalate 5 and sodiumcarbonate, the relative proportions of which will depend upon the excessof sodium carbonate used to ell'ect the decomposition of the calciumoxalate. Thus if we use an excess of 25%, we may obtain a solution inwhich the sodium oxalate is about 80% of the dissolved salts, and sodiumcarbonate about 20%.

'0 may treat this solution to produce a mixture which may be used forthe production of oxalic acid by reaction with sulphuric acid in themanner well known.

In the product-ion of oxalic acid by treatment of an oxalate withsulphuricacid it is advantageous to have the oxalate combined with abase which forms an insoluble salt with sulphuric acid. e may thereforetreat the solution of sodium oxalate and sodium carbonate with calciumhydroxide for example, and obtain a mixture containing a solution ofsodium hydroxide, solid calcium oxalate, and solid calcium carbonate,which upon filtering yields a solution of sodium hydroxide and a cakecontaining calcium oxalate and calcium carbonate. \Ve may now treat thiscake with sulphuric acid for the production of oxalic acid in the mannerwell known and we may utilize the solution of sodium hydroxide for theproduction of sodium carbonate by treating it with carbon dioxide. Thesolution of sodium carbonate thus obtained we may return to the processand use for the treatment of other low grade calcium oxalate mixtures.The composition of the final cake will obviously depend upon excess ofsodium carbonate which is used in decomposing the calcium oxalate of theoriginal mixture. As previously stated, an excess of 19% to 20% issuflicient when the extraction is carried out at 95 to 100 C. Infilterin the mixture, however, some cooling may ta e place, as a resultof-which reversion may occur if only sutficient sodium carbonate hasbeen used to effect the decomposition of the calcium oxa- 50 late at theboiling temperature. Consequently we find it desirable to add suliicicntsodium carbonate to prevent reversio-nduring the filterin operation. Theamount which shouldie added to accomplish this result depends u on thetemperature of the liquid during ltering, and obviously the tempera:'ture will vary with the different types of filtering equipmentemployed. When using uipment, the design of which does not permit thetemperature of the liquid to fall below 85 C., we have found that anexcess of of sodium carbonate will'prevent-re- -version, an excess whichis 15% greaterthan is required to accomplish the decomposition 65 at theboiling temperature.

matic view Assuming, for example, that an excess of 33% of sodiumcarbonate is used in treating the original mixture in the foregoingillustra tion, a simple calculation 'shows that the said originalmixture of 20% calcium oxalate, CaC O,.H,O, and-% calcium carbonate,CaCO may be transformed into a mixture} containin substantially 80%CaC,O,.H. IQ and 9.0% 5100;, and we have consistently obs; t-ained thisresult in practice. i w In the accompanying drawing, constituting? apart hereof, the single figure is a diagrami of a system for theproduction'oi; oxalic acid by our new method.

\Ve first provide a low grade oxalate cake 1, which may be made inaccordance with the; process of the above mentioned copendin applicationor by an othermethod, and w iichfl contains generally about 20% ofcalcium ox-Q' alate. The cake 1 is mixed with water and run into thedigester 2 where it is treated .with a sodium carbonate'sol'ution fromtank 3, in; considerable excess. Generally the excess isfrom 10% to.35%. The mixture is digested at a temperature of about C. forming asolution of sodium oxalatewith an excess of sodium carbonate, thesolublematerial being separated from the insoluble calcium salts i thefilter 4 which is maintained at as high'a a temperature as practical,generally between 80 and 90 C. The resulting precipitatefi is allowed togo to waste. 1 i

The oxalate solution isthen run into the; mixing tank 6 where-itiistreated with milk-" of lime 7 or a crude 'oxalate cake high in free limein order to re-precipitate calcium oxalate 1'; and incidentally-convertthe excess of sodium carbonate to sodium hydroxide. -The mate i rial isthen run into the-filter 8, the filtrate 9 l being mixed with the sodiumcarbonate. solution in tank 3 to make up for losses, and is treated withCO,to regenerate sodium carbon ate for use in the digesteri Theprecipitate 10 contains about'80% of calcium' oxalate and about 20% ofcalcium carbonate which is formed from the excess of sodium carbonate Yused-at the digesting s'tagc of the operation. This enriched precipitateis run into-the digester 11 where it is treated with a sufficient lamount of sulphuricacid to decompose. the oxalate, forming oxalic acid',which is separated from the resulting insoluble calcium sulphate andother calcium compounds in filter 12.

In applying our process to mixtures cor.v taining calcium oxide orcalcium hydroxide in v addition to calcium carbonate we may roceed asfollows :'VVe prepare a slurry of t e mixture with water and addtheretotan amount of 'sodium carbonate suflicient both to preciprtateall the calcium hydroxide as calcium carbonate and to decompose thecalcium oxalateas herein previousl set forth. We now have e mixturewhich differs only from the corresponding mixture of the previouslydescribed m dification in that it cgpteins sod um droxide as one of theproducts of the interaction of calcium hydroxide and sodium carbonate.'e may now proceed 'as setforth in the preceding modification and obtaina solution containing sodium oxalate, sodium carbonateand sodiumhydroxide. This solution we may treat with calcium hydroxide in themanner previously disclosed and obtain a mixture containing calciumoxalate, calcium carbonate, and sodium hydroxide which upon filtering;yields a solution of sodium hydroxide and a cake containing calciumoxalate and calcium carbonate, in which the proportion of oxalate tocarbonate may be as previously stated.

In applying our process to mixtures containing both ca cium carbonateand calcium oxide or calcium hydroxide, we may also proceed asfollows,this modification representng a different method of utilizingcarbon dioxide in our process. \Ve prepare a slurry of the mixture withwater and add thereto an amount.- of sodium carbonate suflicicnt todecompose the calcium'oxalat'e as hereinbeforc set forth. At the sametime we pass CO gas through the slurry. This CC) may either precipitatethe calcium hydroxide as calcium carbonate, or else may transform intosodium carbonate whatever sodium hydroxidehas been formed by theinteraction of calcium. hydroxide with sodium carbonate. In any eventthe CO added serves the purpose of precipitating calcium hydroxide andconsequently makes itunnecessary to add an excess f sodium carbonate tothe slurry for this purpose. he addition of CO is discontinued as soonas all the calcium hydroxide has been precipitated and before anysubstantial portion of sodium carbonate has been transformed into sodiumbicarbonate. We may now proceed as in the first modiliation and obtain asolution containing sodium oxalate and sodium carbonate, which may betrcaled with calcium hydroxide to produce a cake for the manufacture ofoxalic acid.

It will be noted from the above description of our invention that wehave ttllx'Cll a low grade oxalate cake containing. approximately 20% ofoxalate which would require 'a large excess of sulphuric acid. in theneighborhood of 400%, to recompose to oxalic acid. By our simple processof carbonating. filtering and treating with hmc. we

have so enriched the oxalatecake that it requires only a slight excessof sulphuric acid over the theoretical to produce oxalic acid therefromIn large scale operations this represents a great saving in cost ofmaterials. The sodium carbonate, although a relatively expensivematerial, is not lost in the process but is reused and therefore thecost thereof is no important .factor in the cost of operation. As afinal result. we use CO lime and sulphuric acid in our process,the saidmaterials being among the most inexpensive that can be obtained.Furthermore our process does not require the services of exceptionallyskilled operators sincethe steps of the process consist mainly in mixingand filtering operations which can readily be ac complishcd by theordinary unskilled'workers.

\Vhat-we claim is:

1. The method of converting calcium oxalate into an alkali metal oxalatewhich comprises reacting on calcium oxalate with an alkali metalcarbonatcin the presence of water above C., said alkali metal carbonatebcing present in an excess of about 15% to 35%; over the theoreticalrequired, and recovering a solution of alkali metal oxalate.

2. The method of converting calcium oxalate into an alkali metal oxalatewhich comprises reacting on calcium .oxalate with an excess of an alkalimetal carbonate in the presence of water at elevated temperatures, thesaid excess decreasing with increasing temperatures. and recovering asolution of an alkali metal oxalate.

3, The method of converting calcium oxalate into sodium oxalate whichcomprisesreacting on calcium oxalate with sodium carbonate in thepresence of water at elevated temperatures, said sodium carlmnate. beingpresent in an excess of at least 191-1; to 2015 overthe theoreticalrequired. and recovering a solution of sodium oxalate.

1. The method of recovering sodium oxalate from a mixture containingcalcium oxa late and other insoluble calcim n compounds which comprisesforming a slurry of said mixture with water, adding sodium t'ulhun' atein at least-10% excess: of the theoretical required to reactwith thecalcium oxalate present, heating to above 1);) (.7. and rccovcrin}: asolution of sodium oxalate.

5. 'l'hcmethod of tran forming a lo'w grade calcium oxalate mixturecontaining calcium carbonate and calcium hydroxide illw to 1'. highgrade calcium oxalate mixture. which comprises forming a slurry of saidlow grade mixture with water. adding sodium carbonate in an amount inexcess of the theoretical required to react with the calcium oxalate andcalcium hydroxide present, heating and agitating said slurry until thecalcium oxalate has been transformed into sodium oxalate, filtering torecov r a solution of sodium oxalate and other sodium salts. reactingupon last named solution with calcium hydroxide to precipitate theoxalate and carbonate as the calcium salts, filtering to recover a highgrade calcium oxalate mixture. and treating the resulting filtrate withcarbon dioxide to carbonate any sodium hydroxide therein. and returningthe. sodium ca rbouate thus produced to the process.

6. The method of recovering sodium oxalate from a mixture containingcalcium oxalate and other insoluble calcium compounds which comprisesforming a slurry.v of said 5 mixture with water, adding sodium carbonatein at least 10% excess of the theoretical required to react with thecalcium oxalate present, heating to above 40 C. and recovering asolution of sodium oxalate.

In testimony whereof, we have hereunto subscribed our names this 22 dayof August GUY H; BUCHANAN. GEORGE BARSKY.

